COUNTING ATOMS TO PLACE HUMAN IMPACTS IN A GEOLOGIC CONTEXT

Humans, with their tools, brains, and expanding populations have been changing the Earth for millennia – moving soil, cutting trees, damming rivers, and altering the atmosphere. With the advent of fossil fuels, the rate at which people could change the Earth increased dramatically. Critical to understanding the magnitude of human impact and thus to determining what mitigation strategies might make the most sense and be most effective requires data – not only on the magnitude of human impact today but on background rates of change in the past. Cosmogenic nuclides, including Be-10, C-14, and Al-26 can be useful monitors of millennial scale rates of change that help place human impact in the proper geologic context. This talk provides several examples of how cosmogenic nuclides can help contextualize rates of change during the Anthropocene.

Since the 1990s, measurements of cosmogenic nuclides in river sediment have been used to compare contemporary rates of sediment export with background rates of sediment generation - important data for understanding the global flux of sediment and sediment-associated nutrients to deltas and the world’s oceans as well as estimating the filling rates of reservoirs. Where Anthropocene rates of sediment export have been measured, they typically exceed geologic rates of sediment generation by several times. Global compilations and many regional studies show average basin slope is a reasonable proxy for long-term erosion rates providing a simple, calibrated means to estimate rates needed to establish regulatory limits for sediment when it is considered a pollutant.

Since the 1980s, a variety of different nuclides have been used to determine the history of now-vanished glaciers and ice sheets mostly by dating moraines. Recently, in light of rapid, human-induced climate change, cosmogenic nuclides have been used to understand better the sensitivity of ice sheets to past periods of warmth. Measurements of Be-10 and Al-26 in marine sediment cores, ice-bound cobbles, and rock recovered from under ice have been used to argue both for and against the long-term stability of the Greenland and Antarctic Ice Sheets. Such data, once we are better able to interpret it, could well allow us to see into the past and determine how Earth’s ice sheets responded to past periods of exceptional warmth – our likely future.